In this post, we’re going to take a look at the R in CRUD: Retrieve.

We will be using the ruby-oci8 driver to retrieve some data from the database tables, using the connection object created in the Initial Setup section of the first post in this series.

Simple Query

We will perform a simple query that pulls all of the records in no particular order.  Here are the steps we’ll follow in the code snippet below.

1. Create the connection object. We will use this object to perform our database operations.
2. Define the SQL SELECT statement, specifying the columns desired from the table.
3. Create a cursor by parsing the statement.
4. Execute the cursor.
5. Fetch and loop through the rows.
6. Print each row.

# Query all rows
con = OCI8.new(connectString)
statement = 'select id, name, age, notes from lcs_people'
cursor = con.parse(statement)
cursor.exec
cursor.fetch() {|row|
    print row
}

When I run this code in my Ruby session, I see:

[#<BigDecimal:15bcb00,'0.1E1',9(18)>, "Bob", #<BigDecimal:15bc790,'0.35E2',9(18)>, "I like dogs"][#<BigDecimal:15bd1b8,'0.2E1',9(18)>, "Kim", #<BigDecimal:193bf60,'0.27E2',9(18)>, "I like birds"]

This is not real pretty, so from here on, we’ll use printf.

printf "Id: %d, Name: %s, Age: %d, Notes: %s\n", row[0], row[1], row[2], row[3]

Much better:

Id: 1, Name: Bob, Age: 35, Notes: I like dogs
Id: 2, Name: Kim, Age: 27, Notes: I like birds

Extra Fun 1

Modify the statement to order by age.  When you’re done, the results should be:

Id: 2, Name: Kim, Age: 27, Notes: I like birds
Id: 1, Name: Bob, Age: 35, Notes: I like dogs

Answer:

con = OCI8.new(connectString);

statement = 'select id, name, age, notes from lcs_people order by age'
cursor = con.parse(statement)
cursor.exec
cursor.fetch() {|row|
    printf "Id: %d, Name: %s, Age: %d, Notes: %s\n", row[0], row[1], row[2], row[3]
}

Select Specific Rows

Now suppose I only want to see the data for Kim. I want, therefore, to restrict the rows returned by the SELECT. This is done with a WHERE clause. There are several ways to do this.

We could just put the where clause in the statement and it would work.

statement = "select id, name, age, notes from lcs_people where name = 'Kim'"

However, we want to choose the name at runtime and store it in a variable called person_name. You could accept the value in as an argument passed into a function, but we’ll just set a variable to keep it simple.

person_name = 'Kim'

It is possible to simply concatenate the value into the statement.

statement = "select id, name, age, notes from lcs_people where name = '#{person_name}'"

This is very dangerous and opens our code to a SQL Injection attack. You can follow that link for more information, but we won’t be going into detail in this series.  Just know that you should, generally, never allow end user input to be fed directly into a dynamic SQL statement.

A much safer way to pass external values into the SQL statement is by using bind variables with prepared statements.

You have a couple different options:

Positional

statement = "select id, name, age, notes from lcs_people where name = :1 and age = :2"
cursor = con.parse(statement)
cursor.bind_param(1, 'Bob')
cursor.bind_param(2, 35)

statement = "select id, name, age, notes from lcs_people where name = :2 and age = :1"
cursor = con.parse(statement)
cursor.bind_param(1, 'Bob')
cursor.bind_param(2, 35)

Notice the :1 and :2 are switched in the two examples. With a positional statement, the labels do not matter, it could just as well have been :1 and :something. What matters is the first :variable in the statement will be assigned the value of the bind_param with the index of 1, the second the value of index 2 and so on.

Positional bind parameters will us an integer as the key in bind_param().

Named

statement = "select id, name, age, notes from lcs_people where name = :name and age = :age"
cursor = con.parse(statement)
cursor.bind_param('name', 'Bob')
cursor.bind_param('age', 35)

statement = "select id, name, age, notes from lcs_people where name = :age and age = :name"
cursor = con.parse(statement)
cursor.bind_param('age', 'Bob')
cursor.bind_param('name', 35)

With this method, the :name variable will be assigned the value of ‘name’ in the provided key value set.

In the second example, I switched the labels in the SQL, and I also had to switch the keys in the bind_parameter calls so the correct value is still used.

Notice, in both examples, that we do not wrap the bind variable for the name with quotes. This is handled automatically when the statement is prepared for execution.

Example:

1. Create the connection object.
2. Create the person_name variable and assign it a value of ‘Kim.’
3. Define the SQL SELECT statement, specifying the columns desired from the table.
4. Create a cursor by parsing the statement.
5. Bind the value of person_name to :name.
6. Execute the cursor.
7. Fetch and loop through the rows.
8. Print each row.

# Query for Kim
con = OCI8.new(connectString)

person_name = 'Kim'
statement = "select id, name, age, notes from lcs_people where name=:name"
cursor = con.parse(statement)
cursor.bind_param('name', person_name)
cursor.exec
cursor.fetch() {|row|
   printf "Id: %d, Name: %s, Age: %d, Notes: %s\n", row[0], row[1], row[2], row[3]
}

This will return only the data for Kim:

Id: 2, Name: Kim, Age: 27, Notes: I like birds

Extra Fun 2

Modify the statement and variable to get the people older than 30. When you’re done the results should be:

Id: 1, Name: Bob, Age: 35, Notes: I like dogs

Answer:

con = OCI8.new(connectString)

statement = "select id, name, age, notes from lcs_people where age > :age"
cursor = con.parse(statement)
cursor.bind_param('age', 30)
cursor.exec
cursor.fetch() {|row|
    printf "Id: %d, Name: %s, Age: %d, Notes: %s\n", row[0], row[1], row[2], row[3]
}

In this section, we took a look at some basic query functionality.  When you experiment with more complex queries, if you run into problems leave a comment here or on twitter and we’ll find an answer together.

Some things you could try

• Join the lcs_people and lcs_pets table to get the people and their pets.
• Only retrieve the person’s name and age.
• Change the order to display in descending order.

Hint: If you have trouble getting a query to run in your code, try running it in SQL Plus or another database console tool. This will help determine if the problem is with the query or the code.